Some of the protocol layers at the Gb-interface in SGSN contain traffic dependent information, e.g. received/sent sequence numbers. The traffic dependent protocol layers in question are LLC, described in GSM 04.64: “Digital cellular telecommunications system (Phase 2+); General Packet Radio Service (GPRS); Mobile Station—Serving GPRS Support Node (MS-SGSN) Logical Link Control (LLC) layer specification”, version 6.7.0 Release 1997, February 2000, SNDCP described in GSM 04.65: “Digital cellular telecommunications system (Phase 2+);
General Packet Radio Service (GPRS); Mobile Station (MS)—Serving GPRS Support Node (SGSN); Subnetwork Dependent Convergence Protocol (SNDCP)”, version 6.7.0 Release 1997, February 2000, and GMM, described in GSM 04.08: “Digital cellular telecommunications system (Phase 2+);
Mobile radio interface layer 3 specification”, version 6.10.0 (6.a.0) Release 1997, April 2000. The lower most protocol layers at the Gb-interface (BSSGP, NS and L1bis) are stateless, i.e., no MS-related information which changes value during a session are stored in these layers.
If a CPU/processor board with volatile memory serving one or both of LLC/SNDCP protocol layers is restarted, the traffic dependent information is lost in SGSN. Volatile memory is typically dynamic RAM.
According to the GPRS standards, the MS'es which are served by a restarted Gb-interface shall be thrown out of the SGSN-node, as stated in GSM 04.08, Section 13.6.2: “When an SGSN fails, it deletes all MM and PDP contexts affected by the failure”.
The SGSN will not notify the MS about the failure. When the MS eventually has something to send, it will detect that it has been thrown out of the SGSN, and thus must perform a new GPRS attach and PDP context activation.
When SGSN sends and receives LLC and SNDCP PDU'es, the following must be stored; state variables, sequence numbers and frame dependent input to ciphering function. LLC and SNDCP PDU are sent/received at a high rate (throughput). After a processor board failure, several techniques exist to re-establish the communication between MS and SGSN:
Using a hard disk,                i.e., storing traffic dependent information on disk. However, this type of memory has a high latency, and thus will lower the throughput (packets per second), i.e. not very feasible.        
Using static RAM.                I.e., storing traffic dependent information in static RAM (instead of dynamic RAM). Static RAM is more expensive than Dynamic RAM, and thus increases the production cost of a SGSN.        Throw out the affected MS'es from the SGSN, and let them do new GPRS attach, as described in [4] GSM 03.60: “Digital cellular telecommunications system (Phase 2+);        
General Packet Radio Service (GPRS); Service Description; Stage 2”, version 6.7.0 Release 1997, March 2000, Section 13.6.2. This will be perceived as annoying for the subscriber using the Mobile. Mobiles which do not send anything and are within the same Routing Area, will use up to 54 minutes (default periodic routing area update timer interval), ref. GSM 04.08, Section 4.7.5.1 and Table 11.3a, before GMM in the MS detects it has been thrown out. For example, MS terminating SMS-traffic over GPRS might not have been deliverable for 54 minutes. Another example is push services, such as reports from stock-marked which will not be deliverable for thrown out subscribers.